1. Field of the Invention
The present invention relates to an image processing apparatus applicable to an image forming apparatus, such as a copier, a printer, and a facsimile machine, and more particularly, to a growth order of a dither matrix applied to image data in association with dithering as a form of a halftone processing method.
2. Description of the Related Art
As one of types of image data input to an image forming apparatus, there is a continuous-tone image such as a photo image. Such continuous-tone image data is multi-level data of 8 bits to 12 bits per pixel. However, the number of expressible gradation levels per pixel by an image forming apparatus (including an electrophotographic image forming apparatus), which forms an image on a recording medium such as a paper sheet (i.e., outputs an image in so-called hardcopy), is virtually very small. To cope with insufficient gradation levels, such hardcopy equipment converts input image data into a pseudo-halftone image in such a manner that the resolution is enhanced, and an image density is a really modulated on the basis of a plurality of pixels. Image processing performed on input image data in a step of converting the input image data into a pseudo-halftone image is referred to as halftone processing. The present invention relates to a growth order of a dither matrix applied to image data in association with dithering as a form of the halftone processing method (i.e., a method for processing input image data into output image data responding to an output image graduating from low to high densities or an output image in multiple colors).
For details of a quantization process of multi-level image data with the dithering, refer to, for example, pages 51 to 59 of DENSHI SHASHIN GAKKAISHI (Electrophotography), vol. 24, no. 1 (1985). An image subjected to a dithering process has a periodic image structure. As types of dithering, there are mainly (1) clustered-dot dithering (dot screening), (2) Bayer dithering, and (3) line dithering (line screening). Characteristics of these dithering are explained in detail below.
(1) Clustered-Dot Dithering
In a clustered-dot dither matrix, a growth order (the order of pixels to be written from the one having a low image density to the one having a high image density) in a planar direction is the order from a pixel closest to a pixel called a growth center to a pixel peripheral to the growth center. Therefore, in the clustered-dot dither matrix, dots are arranged to be overlapped with other dots so that a dot overlapping area is relatively large (thus, it is possible to shorten a perimeter of a halftone dot composed of some dots). In most of image forming apparatuses (including an electrophotographic image forming apparatus, an inkjet image forming apparatus, and an offset printing machine), it is a fact that a periphery of a dot spreads outside an ideal dot boundary (i.e., a phenomenon called “dot gain” occurs).
Due to the dot gain, an image having a higher density than a target image density is output, which leads to a decrease in gradation reproducibility. However, in the clustered-dot dither matrix capable of expanding a dot overlapping area, it is possible to reduce the effect of the dot gain (because the spread of the dots is offset in the dot overlapping area, so that an image is not subject to adverse effect due to the dot spread).
Therefore, the clustered-dot dither matrix makes possible to output an image with a high gradation reproducibility. Although the clustered-dot dither matrix has such an advantage, the clustered-dot dither matrix has also disadvantages as follows. In the clustered-dot dither matrix, a periodic structure of the growth center needs to be substantially square in shape, so that a possible screen ruling and a flexibility of a screen angle in the dither matrix are disadvantageously low. In a color image forming apparatus that forms a color image by overlapping a plurality of different color screens, these disadvantages lead to an image defect that a false color called color moire appears on the image is prone to occur.
(2) Bayer Dithering
A Bayer dither matrix has a property opposite to that of the clustered-dot dither matrix. In the Bayer dither matrix, dots are dispersed so that the dots are discretely arranged as far as possible. Therefore, the Bayer dither matrix can maintain the resolution high. However, the Bayer dither matrix has a disadvantage of a low gradation reproducibility (this is because it is susceptible to the effect of dot gain). In an old-type low-resolution (of up to 300 dpi) image forming apparatus, the Bayer dither matrix capable of maintaining the resolution high was used. However, nowadays, a high-resolution (of up to 600 dpi or 1200 dpi) image forming apparatus has been widespread, so that there are not many requirements of maintaining the resolution high by the use of the Bayer dither matrix. Thus, the Bayer dither matrix has become seldom used recently.
(3) Line Dithering
In a line dither matrix, a growth order of dots is determined in accordance with the order from a pixel closest to a virtual line called a center line to a pixel farthest from the center line. In the clustered-dot dither matrix, the periodic structure of the growth center needs to be substantially square in shape, so that a possible screen ruling and a flexibility of a screen angle in the dither matrix are disadvantageously low. However, in the line dither matrix, there is no difference between a case where the periodic structure of the growth center is rectangular or parallelogram in shape and a case where the periodic structure of the growth center is square in shape. Therefore, it is possible to increase a degree of freedom of a selection from combinations of a possible screen ruling and a screen angle. Thus, the line dither matrix is effective when a color image is formed by overlapping a plurality of different color screens. When the different color screens are overlapped to form the color image, a moire pattern called color moire may appear on the color image depending on a combination of a possible screen ruling and a screen angle, which leads to an image defect of the color image. To prevent the color moire from appearing, the different color screens are generally set at an angle with one another (set to have an angular difference of at least 20 degrees between them). For example, when a full-color image is formed, to reduce the color moire, screen angles of cyan (C), magenta (M), yellow (Y), and black (K) color screens are individually set so that an angular difference between them can be as large as possible. When the line dither matrix is used, as described above, there are various combinations of a screen ruling and a screen angle. In regard to this point, the line dither matrix is more favorable than the clustered-dot dither matrix.
Furthermore, in the line dither matrix, there is one directional axis (i.e., the line dither matrix has the periodic structure only in one direction), so that four directional axes of the C, M, Y, and K color screens can be set in a range of 0 to 180 degrees. Thus, in the line dither matrix, color moire is less prone to appear. Incidentally, in the clustered-dot dither matrix, there are two directional axes, so that directional axes of the C, M, Y, and K color screens need to be set within a range of 0 to 90 degrees, and thus color moire is prone to appear. For such reasons, by the use of the line dither matrix, a fine image that color moire is suppressed can be obtained easily. Although the line dither matrix has such advantages, a gradation reproducibility is lower than that of the clustered-dot dither matrix. This is because in the line dither matrix, as compared with the clustered-dot dither matrix, dots are discretely arranged especially in an area where a dot-area ratio is small such as a highlight portion, so that it is susceptible to the effect of dot gain. Especially, in a case of an electrophotographic image forming apparatus, if dots are discretely arranged, an exposure energy is dispersed when a photosensitive element is exposed to a laser beam to form an electrostatic latent image on the photosensitive element. Thus, the highlight portion may be formed in off-color. Namely, when the line dither matrix is used in the electrophotographic image forming apparatus, it is difficult to express gradation of color in the highlight portion.
By the way, a conventionally well-known point-of-purchase advertising (hereinafter, “a POP image”), such as an outdoor advertising signboard of a retail store, a poster, and a price card attached to a wall or a ceiling in a store, is recognized as a type of image different from types of a text image, a graphic image, a photo image, and the like. For example, Japanese Patent No. 3728614 discloses a method for outputting a POP image with stable hues in combination with a specific developing method. Furthermore, Japanese Patent Application Laid-open No. H11-15336 discloses an image forming apparatus capable of forming an image without decrease in image density even when an output image is a high-density image such as a POP image.
Moreover, although it is not the invention relating to a POP image, Japanese Patent Application Laid-open No. 2001-53978 discloses a technique for correcting image data depending on image information (information on a type of image, for example, information indicating that an output image is a graphic image) from an external device (a printer driver).
Specifically, Japanese Patent No. 3728614 discloses a nonmagnetic mono-component contact development method. In this method, at least two toners are arbitrarily selected from C, M, Y, K, and white toners, and the selected toners are mixed to be used for development. A potential difference between a potential of a latent image on an image carrier as an object to be developed and a potential of a developing bias applied to a developing roller is set to be equal to or larger than a maximum potential difference among potentials to be respectively saturated by a development concentration when each of the toners before being mixed is subjected to the development, and the development is performed with the mixed toner.
By such a configuration, even when a mixed toner obtained by mixing any of the C, M, Y, K, and white toners is used for development, a selective development does not occur. Therefore, even in a continuous printing of an image of a constant hue with a mixed toner in a desired color created by mixing commonly-marketed cheap color toners, it is possible to print the image with a stable hue.
When there is a request for printing a POP image although not in full-color but in specific colors, the toners used to be customized conventionally. However, according to the invention disclosed in Japanese Patent No. 3728614, it is not necessary to customize the toners, and it is possible to output the POP image with stable hues.
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. H11-15336 includes a control unit that outputs a replenishment instruction to a replenishing unit. Upon receipt of the replenishment instruction from the control unit, the replenishing unit replenishes a toner to a developing unit. A mode setting unit informs the control unit whether an image of an original is a high-density image. Furthermore, when a second comparing unit detects that a count number becomes equal to a cycle-out number that is set in advance, the control unit suspends a job, and clears the cycle-out number. Moreover, the control unit outputs a job resumption instruction or a toner replenishment instruction to the replenishing unit depending on a result of determination by a concentration comparing unit.
By such a configuration, the image forming apparatus disclosed in Japanese Patent Application Laid-open No. H11-15336 can solve the conventional problem that a toner concentration is decreased due to the lack of a toner when an image having a higher density than that of a general document, such as a POP image, is formed because a regularly-supplied toner amount is insufficient with respect to a toner amount used in development.
A color image processing apparatus disclosed in Japanese Patent Application Laid-open No. 2001-53978 includes a graphic processing unit and a graphic concentration correcting unit. When image information received from the external device indicates that input image data is a graphic image, the graphic processing unit processes the input image data. The graphic concentration correcting unit corrects a concentration of the graphic image. When the concentration of the graphic image to be processed is low, the graphic processing unit causes the graphic concentration correcting unit to correct color information so as to change the low concentration to a reproducible concentration.
In a conventional configuration, a color of a line thinner than a dither pattern cannot be expressed faithfully, and also the line cannot be drawn at all. These are serious problems for an application using a thin line frequently, such as a computer-aided design system (CAD). However, the color image processing apparatus disclosed in Japanese Patent Application Laid-open No. 2001-53978 can process image data without losing a line image in thin line processing, and also can reproduce the line image.
Japanese Patent Application Laid-open No. 2007-025307 discloses an image forming apparatus. The image forming apparatus includes an image processing apparatus and an image forming unit. The image processing apparatus includes a halftone processing unit that converts multicolored image data into pseudo-halftone image data. The image forming unit includes a photosensitive element and a developing unit capable of developing a latent image into a color toner image with any of a plurality of color toners. The image forming unit forms a color image in such a manner that a plurality of different color toner images are superimposed on the photosensitive element, the superimposed toner images on the photosensitive element are collectively transferred onto a transfer medium. The halftone processing unit performs halftone processing (a multi-level quantization process), specifically, quantizes the image data at a quantization level of 3 or more bits on a pixel-by-pixel basis. Furthermore, the halftone processing unit causes all pixels in a high-density area to grow averagely in accordance with a growth order (causes all pixels in a high-density area to grow uniformly).
By such a configuration, the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2007-025307 can solve the conventional problem in superimposing toner images on the photosensitive element that “when the second color toner or the subsequent color toner is supplied for development, a toner adhesion amount is significantly different between a toner layer area and a non-toner layer area on the photosensitive element” by a different method from conventional methods.
However, there has not been developed a halftone processing technique by the application of any dither matrix characterizing a POP image.
In halftone processing based on an area coverage modulation generally-employed in hardcopy equipment including an electrophotographic image forming apparatus, an image density is a really modulated on the basis of a plurality of pixels, and input image data is converted into a pseudo-halftone image (graduations from a low density to a medium density and from the medium density to a high density can be reproduced by changing a ratio of a toner adhesion area to a non-toner adhesion area).
Therefore, in the halftone processing based on the area coverage modulation, for example, when a toner adhesion amount is controlled to be smaller than a maximum adhesion amount, a proportion of the toner adhesion area is reduced, thereby increasing a proportion of a white background portion (an area where a portion of a paper sheet is exposed).
However, when an output image is a POP image, the method that the toner adhesion amount is controlled by increasing the proportion of the white background portion leads to the following problems. A POP image puts emphasis on an image quality of an area having a uniform density (generally, called “a solid patch”). As a typical color of the solid patch, in addition to C, M, Y, and K colors, high-density colors of red (R), green (G), and blue (B) are major examples. Other colors located in an outermost ward of a color reproducible range (the color gamut) also fall under the color of the solid patch. Why a POP image puts emphasis on an image quality is concretely described below. An area having a uniform density in a POP image has to be entirely covered with a toner image, i.e., a surface of a paper sheet is strongly required not to be exposed. Therefore, if such an area where the surface of the paper sheet is exposed (or a lower-level toner layer is exposed), as is common in the area coverage modulation, is formed in the solid patch, a user has an impression that “the solid patch is not colored finely”, which results in a factor of user dissatisfaction.
When an image having an area where the surface of the paper sheet is exposed by the area coverage modulation is formed, the reason why a user is dissatisfied with a formed image is not sure. The inventors of the present application speculate the reason why user is dissatisfied with the formed image is that a dot-like or line-like structure (a periodic structure in a dithered image) appears in a portion supposed to be a uniform density.
If all colors can be expressed while preventing the surface of the paper sheet from being exposed, it can be expected that a user would not feel such dissatisfaction that “the solid patch is not colored finely”. However, in the halftone processing based on the current area coverage modulation, if it is configured not to expose the surface of the paper sheet, it is not possible to adjust a toner adhesion amount. Thus, it is not possible to reproduce a color reproduced by changing a toner amount subtly (especially, a color reproduced by a combination of at least two different color toners, such as slightly-yellowish red that is reproduced by reducing an amount of M-toner slightly to be smaller than that is required to reproduce red).
Furthermore, to achieve output image in the same color among different models of image forming apparatuses, each of the image forming apparatuses needs to change a toner amount subtly to reproduce the same color. This is because, for example, a spectral reflectivity of toner to be used and the order of toner images to be formed on a paper sheet are different depending on the models. Therefore, even when different models of image forming apparatuses intend to reproduce the same color, a toner adhesion amount is different depending on the models. Thus, to reproduce the same color among different models of image forming apparatuses, each of the image forming apparatuses needs to change a toner amount subtly to adjust the color.
In a conventional technology, such a subtle change of toner amount is generally made by performing the area coverage modulation. In other words, in the halftone processing based on the current area coverage modulation, when it is controlled not to expose the surface of the paper sheet, a toner amount for the same image cannot be adjusted. Thus, it is not possible to reproduce the same color among different models of image forming apparatuses.
Moreover, in the area coverage modulation, a toner adhesion amount is controlled by producing a white background portion. However, according to a review of this method by the inventors of the present application, a white light comes to be mixed into a reflected light from a toner image, so that a reproducibility of a high-saturation and high-brightness color (i.e., a vivid and bright color) is poor. An impressive POP image is preferred, so that the poor reproducibility of the high-saturation and high-brightness color is a major disadvantage.